This comparative study was completed with the examination of the trajectory of the dorsal component of the lateral olfactory tract (dLOT) through the accessory olfactory formation in squirrel monkey. A review of over 40 mammals revealed that in some species the dLOT runs between the large output neurons of the accessory olfactory formation and the internal granule cells, while in others it passes below the internal granule cells. A grouping of the animals according to this criterion leads to the same classification of placental mammals as a taxonomy based on bones and teeth. In carnivores and ungulates (both odd and even toed hoofed animals) the dLOT passes below the internal granule cells, while in rodents, edentates, lagomorphs, insectivores and primates, the dLOT separates the internal granule cells from the large output neurons. The results demonstrate that comparative studies of brain structures provide inferences regarding the ancestry of the orders of mammals.